Voltage control oscillators (VCO) play a key role in a number of applications which require a variable frequency source, such as phase locked loops, and the like. In an exemplary phase lock loop (PLL), a VCO might be used as an oscillator that produces a variable frequency as controlled by a filtered output from a phase detector. A phase lock loop is a device that is capable of being programmed to generate a desired frequency. Once it approximates the frequency, the frequency is divided down to a value of a reference frequency, provided by an external oscillator, and compared to that reference frequency. When the difference between the reference frequency and the generated frequency reaches zero, the phase lock loop stops tuning and locks to the frequency that it has just produced. The frequency reference used to tune the phase lock loop is typically provided by a single frequency oscillator circuit. Alternatively, a series of multiple oscillators may be used to provide a range of frequencies. However, the multiple oscillator approach would not provide a continuously variable frequency output.
A VCO, typically found in a PLL, generates signals that are phased locked to a reference frequency. Thus, the VCO must provide an output signal phase locked to the reference frequency.
VCOs contained in PLLs are also required to produce a stable reference frequency. A low phase noise present in the VCO output is desirable to produce a stable output.
VCOs typically accomplish a change in frequency by changing a value of capacitance in the VCO circuitry. Capacitors may be discretely switched in by mechanical switches, or may be a continuously variable mechanical type, or as is typically the case, a variable capacitance is provided by a device that provides a capacitance proportional to an applied voltage. In a given circuit, variations and component values will require an adjustable capacitance that will compensate for a component spread to provide a variable capacitance that allows a desired range of tuning frequencies to be produced by the VCO. The effects of component spread are magnified when a VCO is disposed on an integrated circuit substrate. Components constructed on an integrated circuit substrate typically have wide variations and component values. Thus, the variable capacitance devices used to tune voltage control oscillators are required to have a wider tuning range of capacitances to compensate for the spread in integrated circuit component values.
Those having skill in the art would understand the desirability of having a voltage control oscillator capable of being integrated on an integrated circuit substrate that does not have the problem of requiring a large variable capacitance tuning range. This type of device would necessarily provide higher integrated circuit yields by providing a VCO requiring less of a variable tuning voltage range, thus allowing a tunable VCO to be economically integrated onto an integrated circuit.